Conventionally, it is known that a gas sensor employs a metal-oxide semiconductor, such as tin oxide (SnO2) as a sensitive layer, and is capable of (a) detecting whether or not a gas to be detected is present, or (b) detecting a concentration variance thereof by means of a change in electrical characteristics (e.g., change in resistance) of the metal-oxide semiconductor. The metal-oxide semiconductor used in the gas sensor has a characteristic in which the number of conduction electrons in the metal-oxide semiconductor decreases and the resistance increases due to adsorption of the negative charge of oxygen (O2−) in the atmosphere on to the surface of the metal-oxide semiconductor. In such a state, if any reducing gas (such as carbon monoxide) is present as a gas to be detected in a measurement atmosphere, O2− adsorbed on to the surface of the metal-oxide semiconductor will be desorbed, thus decreasing the resistance of the metal-oxide semiconductor. Based on such change in resistance of the metal-oxide semiconductor, the gas sensor may detect the gas to be detected. That is, the sensitivity of the gas sensor may be indicated by the resistance ratio of the metal-oxide semiconductor according to a presence or absence of the gas to be detected.
Since the metal-oxide semiconductor is susceptible to humidity, the sensitivity of the gas sensor tends to deteriorate under the influence of humidity. If the humidity increases in a measurement atmosphere, the quantity of moisture in the measurement atmosphere adsorbed as hydroxyl group OH− on to a site where O2− should be adsorbed will increase. As a result, an adsorption quantity of O2− on to the surface of the metal-oxide semiconductor will decrease. Therefore, the resistance of the metal-oxide semiconductor in the measurement atmosphere in which the gas to be detected is absent becomes small, when it should normally be high. Moreover, if the humidity in the measurement atmosphere is high, the amount of adsorption of hydroxyl group OH− will increase, thereby reducing the quantity of O2− adsorbed. Furthermore, hydroxyl group OH− adsorbed on to the metal-oxide semiconductor is not desorbed by the gas to be detected. Even if the gas to be detected is present in the measurement atmosphere, the resistance of the metal-oxide semiconductor will not be small, but large. Thus, when the humidity in the measurement atmosphere increases, the sensitivity of the gas sensor deteriorates.
Therefore, by adding (1) a quinquevalent transition metal, such as vanadium, niobium and tantalum, and (2) a trivalent transition metal, such as chromium, to the metal-oxide semiconductor which serves as a gas detection body (sensitive layer), a binding force between O2− and the surface of the metal-oxide semiconductor becomes larger than that of hydroxyl group OH− and the surface of the metal-oxide semiconductor. Consequently, the humidity resistance of the gas sensor may be improved. (e.g., see Japanese Patent Application Laid-open (kokai) No. 2001-305089 (hereinafter “the '089 patent document”))
Problems to be Solved by the Invention
In a gas sensor disclosed in the '089 patent document, only a seasonal dependence under a relatively low load is employed when evaluating the humidity resistance. Humidity resistance under high temperature and high humidity atmosphere (e.g., 60° C., 95% Relative Humidity(RH)), which is required for an in-vehicle gas sensor, is not taken into consideration.
The present invention has been conceived to solve the problems thus far described and has an object to provide a gas sensor having excellent humidity resistance, even if used in a high temperature and high humidity atmosphere.